The aim of our study was to characterize brain dynamics of affective modulation of somatosensory processing in chronic pain. We hypothesized that chronic pain patients will show abnormal EEG activity under negative mood conditions compared to healthy controls. Nineteen patients with chronic pain and 21 healthy subjects participated in the experiment. Multiscale entropy, fractal dimension, event-related potentials, and fast Fourier transform were used to analyze EEG data. A significant enhancement of entropy was found in pain patients at P4 compared to P3. Analysis of fractal dimension also revealed significantly higher values at P4 than P3 when pain patients were viewing unpleasant pictures. By contrast, no significant differences due to hemisphere or affective condition were found on nonlinear measures for healthy controls. Analyses of somatosensory ERPs showed that P50 amplitudes elicited by pleasant pictures were more reduced in chronic pain patients than in healthy controls. Finally, we observed that EEG band power was lower in pain patients than in healthy controls, in particular for theta and beta bands over sensorimotor cortices and temporal regions when viewing pleasant images. These findings suggest that sustained pain seems to be accompanied by an abnormal activation and dynamic of brain networks related to emotional processing of somatosensory information in chronic pain. Furthermore, our findings suggest that both linear and nonlinear measures of EEG time series may contribute to the understanding of brain dysfunction in chronic pain.

In this case study the electroencephalographic (EEG) activity of a 23 years old snake phobic patient was recorded one week before treatment, one week after successful one-session exposure therapy, and one year later. EEG recordings were obtained at rest and during exposure to pictures of snakes, pictures of equivalent arousing power, and emotionally neutral images, all of them taken from the International Affective Pictures System. Measures of brain dynamics were sample entropy (SampEn) for each EEG signal/channel and phase synchronization between pairs of EEG channels. Results showed dramatic changes in both measures one week after treatment: SampEn increased and phase synchrony decreased at all sites and pairs of channels respectively. At follow-up, however, we found patterns of entropy and synchrony change across conditions that were similar to the pre-treatment ones, while the patient did not report any fear at all. Despite the limitations of single case studies, these results suggest that the exposure-induced changes in EEG entropy and synchronization are large but transient. The transient increase of the brain’s flexibility could be one of the working neurophysiological mechanisms of exposure therapy.